An electricity supply system is known which includes: a power feeding means; a power receiving means for contactlessly receiving power from the power feeding means; an efficiency sensing means for sensing an efficiency of transmission between the power feeding means and the power receiving means; a determination means for determining whether or not the sensed efficiency of transmission is above a prescribed value; and a control means for determining that normal electricity supply is prevented by an obstacle, etc., in response to a condition that the sensed transmission efficiency is below the prescribed value, and temporarily suspending the power feeding of the power feeding means, and restarting power feeding by a small power a prescribed time period after the suspending (see patent document 1).
However, since the efficiency of transmission can be lowered when the power feeding coil and the power receiving coil are deviated in position from each other, it has been impossible to detect on a basis of change in the efficiency of transmission a foreign object coming into between the power feeding coil and the power receiving coil.
It is a problem to be solved by the present invention to provide a contactless electricity supply device that detects a foreign object coming into between a power feeding coil and a power receiving coil.